1. Field of the Invention
This invention relates to a method and apparatus for measuring the velocity of a fluid, and more particularly to a method and apparatus for measuring the velocity of a fluid in which a double-exposure image of the speckle pattern or particle images of tracer particles in a fluid is irradiated with a collimated beam of coherent light, and is subjected to Fourier transformation through a lens to produce at its focal plane Young's fringes which are analyzed to determine the fringe spacing and direction in order to measure the velocity of the fluid.
2. Description of the Prior Art
Methods of measuring the two-dimensional velocity component of a fluid include the laser speckle method described by T. D. Dudderar and P. G. Simpkins in "Laser speckle photography in a fluid medium," Nature Vol. 270 3 page 45 (November 1977), and the white-light speckle L method described by E. Bernabeu, J. C. Amare and M. P. Arroyo in "White-light speckle method of measurement of flow velocity distribution," Applied Optics Vol. 21 No. 14, 2583 (1982). These methods utilize the fact that when a fluid is irradiated by a beam of light, the speckle pattern produced by particles in the fluid or the particle image pattern exhibits a translational motion.
The working principle of these methods is to record a speckle or particle image pattern from a fluid flow as a double-exposure image, and the amount of translational movement of the pattern is detected quantitatively by optical processing, using a laser beam. In this case, the displacement of the pattern will correspond to the flow velocity.
A pulsed laser light source or a chopped continuous light source is used to record the double-exposure image, and tracer particles are mixed into the fluid to scatter the light. The fluid containing these tracer particles or scatterers is then irradiated twice by a pulsed beam, and movement of an image of the particles themselves or movement of the speckle pattern is photographed to produce a double-exposure image.
The Young's fringes method is one method used for optical processing of the double-exposure image. When a double-exposure image is irradiated by a collimated beam of laser light, these fringes are produced in the diffracted image of the light passing through a lens and can be observed at the focal plane of the lens. The fringes include information on the velocity of the pattern irradiated by the laser beam and the direction of the normal of the fringes shows the direction of the velocity vector. If the spacing between fringes is d, the wavelength of the irradiating light is .lambda., the focal length of the lens is f, the magnification at which the double-exposure image is photographed is M and the interval between light pulses is T, then the velocity v of a fluid will satisfy the following relation: EQU v=(f.lambda.)/(Mdt).
The flow velocity can be obtained by optically processing the double-exposure image and analyzing the fringes. Using manual methods to find the spacing and direction of the fringes derived from each point of the double-exposure image is extremely time-consuming and inefficient. One recent solution is to display the fringes on a screen, use a TV camera to record the fringes on the screen and find the velocity using a method based on image-processing techniques. This is described in "Automatic fringe analysis with a computer image-processing system," by D. W. Robinson, Applied Optics, Vol. 22, No. 14, 2169 (1983).
However, image processing increases the complexity of the system, establishing the fluid flow velocity requires considerable time, and overall, the system becomes costly.